Various video coding systems can be designed to support parallel entropy coding and entropy decoding processes for different segments of video, for example, slices, tiles or blocks. As one of the final stages of video coding, coded data from different spatial areas of a frame is formed into data sequences and entropy-coded as a string of bits. Early entropy coding techniques had been serial. A coding context carried from bit to bit, down each sequence, and then to the beginning of a next sequence. Until the entropy coding was undone serially, a decoder could not perform any parallel processing of constructs within the entropy-coded sequence.
Wavefront Parallel Processing (“WPP”) introduced the idea of selecting some or all of the sequences to get their entropy context from an initial portion of a previous sequence rather than from an end portion of the previous sequence. By developing the context of a given sequence from the start of the preceding sequence, parallel entropy decoding of the second sequence could be performed once decoding of the first sequence developed a decoding context for the second sequence. Thus, WPP supports parallel processing of the sequences to some degree.
The WPP technique, however, has certain consequences. Parallel decoding of sequences cannot be performed until sequence start points have been identified and an appropriate context has been developed for each sequence. Because the context of a current sequence is developed by entropy decoding a relevant portion of a previously-coded sequence, the WPP introduces dependencies among the sequences. Moreover, because the entropy-coded data is a serially coded bitstream, positions of the various sequences must be identified by an index field table that specifies start points of the sequences.
In the current design of the forthcoming HEVC coding standard, it has been proposed to provide an index in front of the entropy-coded data that identifies the bit-positions of these start points. This causes significant delay, however, because an encoder must buffer all coded video data to be represented by the table, build the table and add it to a coded bitstream as a position that precedes the coded data itself. Essentially, an encoder may start transmitting coded video data of a segment to which the table applies only after the segment is coded in its entirety.
The inventors perceive a need in the art for an entropy coding protocol that supports parallel-processing and yet avoids the latencies associated with prior solutions.